Home' RTCA Documents for Review : DO-230H FRAC Contents 178
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Transmitting uncompressed video over an IT network is not a practical solution. Video compression is
normally done using one of the ITU standard MPEG codecs, the most generally available being the MPEG-
4 AVC which is also an ITU standard, H264, which offers high quality compression with lower bit rates.
In the past, video storage costs could easily exceed video camera costs. That put pressure on ways to reduce
video storage by varying encoder bit rates, image resolution, frame rate, and storage duration. Hard disk
storage has since become so economical that the user can trade off these variable according to the airport's
needs, to be established during the ConOps long with network transmission bandwidth requirements
At the present time, there are no federally-mandated requirements for video resolution or storage duration.
The parameters are left to the airport to decide and specify. The factors to be considered include:
Resolution - The available Standard Definition options include CIF, 2CIF, and 4CIF, the latter
being considered full video. Megapixel cameras have higher resolution imagery and have become
so popular as to dominate video system designs. Resolution can also be varied by the type of camera
and its function, e.g., adopting a lower resolution for general exterior surveillance, especially when
video analytics are also used, but adopting higher resolutions for areas where law enforcement
issues might arise and forensic quality imagery is desirable.
Frame Rate - The standard NTSC frame rate is 30 frames per second (fps), and this is desirable
where object motion is common and where moving object tracking is required. At any particular
time, and especially during periods of low activity, full frame rate video will be unnecessary. For
many surveillance applications, motion is either infrequent or can be observed, and stored, at lower
frame rates. The design should consider what each application requires and whether frame rates of
15, 7, and even 1 fps might suffice. Many analytical functions have minimum frame rate
requirements which will drive the design choice
Bit rates – Digitized video is transmitted over IP networks and stored, and the rate of data flow is
called the bit rate. Many factors affect the bit rate including the compression algorithm, resolution
and frame rate, level of scene activity, amount of PTZ camera usage and lighting levels. Bit rates
can vary during the day, and are susceptible to surges at night in the presence of point light sources
such as headlights. One solution to this variable bit rate is to use a constant bit rate or a capped
variable bit rate, but capping the bit rate may degrade image quality during those periods where the
bit rate needed is higher than the maximum configured.
Flow Control - A major benefit in storage costs can be realized if, during periods of inactivity, little
of a low frame rate video is transmitted provided, however, that as soon as activity is detected,
based on changes to as few as 2 to 4 pixels, normal video operation can be instantly resumed to
ensure that important information is not lost. This is not the same as event-only video, which
typically applies to activity within selected Regions of Interest (ROIs) specified by the user rather
than to pixel changes anywhere in the scene.
Event-Only Video - There will be many surveillance areas where airport security is only interested
in recording activity in regions of interest (ROIs) in the surveillance scene. Storage costs can be
optimized based on probabilities of event activity rather that recording continuous video.
Storage Duration - It is common to store surveillance camera video for a period of 30 days, but this
is neither a standard nor a requirement. Duration should be determined by studying the requirements
of departments within the airport which have a stake in how and for what period video is stored,
and how these requirements should be implemented across different surveillance sites. Some
systems lock the alarm video clips on the Network Video Recorder (NVR) for long periods but it
is important to realize that this will reduce the retention period for the regular video on that NVR.
To avoid this problem archive to a separate, dedicated recorder. Another alternative for long term
retention is to carefully degrade the video after a certain period, so that it consumes less space.
Examples of degrading include dropping frames or reducing resolution.
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